The background of the invention will be discussed in two parts:
1. Field of the Invention
This invention relates to aquarium heaters, and more particularly to an electrical temperature responsive aquarium heater.
2. Description of the Prior Art
In aquariums having tropical or exotic fish, control of the water temperature within a very narrow range is desirable. Such water temperature control is generally accomplished by means of an aquarium water heater which includes an immersible glass tube adapted to be supported by a wall of the aquarium housing and suspended therefrom with the glass tube immersed in the water. The glass tube contains a heater element adjacent the bottom thereof in thermally conductive relation with the glass tube for transmitting heat to the water. Also included within the tube is a suitable temperature responsive device for controlling the transmission of alternating current power to the heater element, such a device usually including a bimetallic element adjustable by means of a knob or the like extending externally of the tube.
Another aquarium heater is shown in U.S. Pat. No. 3,896,289 which utilized a bi-directional thyristor in series with the heater element with a zero voltage switch which electrically actuates the triac or bi-directional thyristor. Control of the triac permits or halts the flow of all current to the heater. A negative temperature coefficient thermistor is utilized to sense the water temperature, the thermistor being suitably encased for positioning within the water externally of the glass tube housing.
It is an object of the present invention to provide a new and improved aquarium water heater utilizing silicon-controlled rectifiers to energize the heater element.
It is another object of the invention to provide a new and improved aquarium water heater control circuit in which one silicon-controlled rectifier provides power to the heater element on one-half the alternating current cycle while simultaneously charging a capacitor, the capacitor discharging on the other half of the alternating current cycle to render a second silicon-controlled rectifier conductive to energize the heater element.